Wikipedia
Micropolyphony is a kind of polyphonic musical texture developed by György Ligeti and then imitated by some other twentieth-century composers, which consists of many lines of dense canons moving at different tempos or rhythms, thus resulting in tone clusters vertically. According to David Cope, "micropolyphony resembles cluster chords, but differs in its use of moving rather than static lines"; it is "a simultaneity of different lines, rhythms, and timbres" .
Differences between micropolyphonic texture and conventional polyphonic texture can be explained by Ligeti’s own description:
The earliest example of micropolyphony in Ligeti's work occurs in the second movement (mm 25–37) of his orchestral composition Apparitions . His next work, Atmosphères for orchestra, the first movement of his later Requiem, for soprano, mezzo-soprano, mixed choir, and orchestra, the unaccompanied choral work Lux aeterna, and Lontano for orchestra, also use the technique. Micropolyphony is easier with larger ensembles or polyphonic instruments such as the piano , though the Poème symphonique for a hundred metronomes creates "micropolyphony of unparallelled complexity" . Many of Ligeti's piano pieces are examples of micropolyphony applied to complex "minimalist" Steve Reich and Pygmy music derived rhythmic schemes.